In production of a semiconductor integrated circuit, an exposure apparatus has been widely utilized which transfers a fine circuit pattern drawn on a mask original plate on a wafer. Along with high integration and high functionality of an integrated circuit, an integrated circuit becomes finner. And an exposure apparatus is required to form an image of a circuit pattern with high resolution on a wafer with a long focal depth, and shortening of the wavelength of the exposure light source is being advanced. The exposure light source has been shifted from conventional g-line (wavelength: 436 nm) to i-line (wavelength: 365 nm), to a KrF excimer laser (wavelength: 248 nm) to an ArF excimer laser (wavelength: 193 nm).
A synthetic quartz glass has been mainly employed as an optical member for an exposure apparatus employing a light having a wavelength of from 170 to 400 nm as a light source, since it is excellent in transparency to light over a wide range from a near infrared region to an ultraviolet region, it has an extremely small thermal expansion coefficient and it is relatively easily processed. As a synthetic quartz glass conventionally employed as an optical member of an exposure apparatus, for example, one as disclosed in JP-A-3-88742 has been known. That is, a synthetic quartz glass having a OH group content of at least 10 ppm and containing hydrogen in an amount of at least 5×1016 molecules/cm3 has been known. When a synthetic quartz glass is irradiated with ultraviolet lights, paramagnetic defects such as E′center (≡Si·) and NBOHC (≡SiO·) are formed. Such paramagnetic defects have optical absorption bands centered at a wavelength of 220 nm and at a wavelength of 260 nm, respectively, and cause a decrease in the light transmittance over a wide wavelength range of from 180 to 300 nm.
Hydrogen molecules in the synthetic quartz glass play a role in converting the E′center and the NBOHC induced by irradiation with ultraviolet lights into ≡SiH and ≡SiOH, respectively, having no absorption band centered at a wavelength range of from 190 to 300 nm. ≡SiH and ≡SiOH do not have an optical absorption band centered at a wavelength range of from 170 to 300 nm, and thus the decrease in the transmittance due to irradiation with ultraviolet lights is suppressed. In the above JP-A-3-88742, attention is paid to the defect restoration effect of hydrogen molecules, and it relates to a method of suppressing the decrease in the transmittance of the synthetic quartz glass upon ultraviolet lights irradiation.
However, when the synthetic quartz glass is irradiated with ultraviolet lights, not only the decrease in the light transmittance but also phenomena called compaction and rarefaction occur. The compaction is such a phenomenon that by irradiation with ultraviolet lights, the density of the synthetic quartz glass at the irradiated portion increases, and along with this change in the density, the refractive index of the synthetic quartz glass at the irradiated portion increases. On the other hand, the rarefaction is such a phenomenon that by irradiation with ultraviolet lights, the density of the synthetic quartz glass at the irradiated portion decreases, and along with this change in the density, the refractive index of the synthetic quartz glass at the irradiated portion decreases. Whether either phenomenon of compaction and rarefaction occurs depends on the type of the synthetic quartz glass or irradiation conditions (energy density, accumulated irradiation energy amount) (C. K. van Peski, et. al., “Behavior of fused silica irradiated by low level 193 rm excimer laser for tens of billions of pulses”, J. Non-cryst. Solids, 265, pp. 285-289 (2000)).
Particularly with respect to the energy density, compaction occurs in a case of irradiation at a pulse energy density of at least 0.05 to 0.1 mJ/cm2/pulse, and rarefaction occurs in a case of irradiation at a lower energy density in general. In a semiconductor exposure apparatus, a fine pattern on a photomask is microtransferred on a water, and accordingly a projection lens is required to have an extremely high uniformity of the refractive index. An overall or local change in the refractive index of the projection lens by irradiation with ultraviolet lights exerts bad influences such as focus position displacement, and makes transfer of the required pattern impossible. Further, if the density of the synthetic quartz glass at an irradiated portion is changed by irradiation with ultraviolet lights, a stress is included in the irradiated portion and a portion peripheral to the irradiated portion, and birefringence of the synthetic quartz glass changes. This change in the birefringence also exerts bad influences over image-formation performance of the projection lens, such being problematic.
The cause of the change in the density of the synthetic quartz glass by irradiation with ultraviolet lights is not clearly understood at present, however, several improvement methods have been proposed.
For example, JP-A-11-116248 proposes a process to incorporate fluorine at a concentration within a range of from 10 to 10,000 wtppm into a synthetic quartz glass and to make the synthetic quartz glass contain substantially no chlorine, thereby to suppress the compaction of the synthetic quartz glass upon the ultraviolet light irradiation. However, fluorine has influences over the refractive index of the synthetic quartz glass, and addition of fluorine in an amount of 1 wtppm to a synthetic quartz glass decreases the refractive index at 633 nm by about 4×10−7. Although the compaction of the titan doped synthetic quartz glass is suppressed upon the ultraviolet light irradiation, it is very difficult to obtain uniformity of the refractive index. However, a synthetic quartz glass to be used for a lens material of a semiconductor exposure apparatus is required to have a uniformity of the refractive index of at most 2×10−5. Accordingly, it is very difficult to apply a titan doped synthetic quartz glass to a lens material of a semiconductor exposure apparatus.
Further, JP-A-2000-191329 proposes a process for producing a synthetic quartz glass with a small amount of compaction. The production process disclosed in JP-A-2000-191329 is such that a porous quartz glass body synthesized by a VAD method is subjected to a heat treatment in an oxidizing atmosphere and then formed into a transparent glass, and the synthetic quartz glass block formed into a transparent glass is subjected to a heat treatment in a hydrogen gas-containing atmosphere to dope the quartz glass with hydrogen molecules. The synthetic quartz glass obtained by this process contains no oxygen deficient defects (≡Si—Si≡), and provides a small amount of compaction upon the ultraviolet light irradiation. According to this process, a synthetic quartz glass with a small amount of compaction can be obtained without impairing other characteristics such as uniformity of the refractive index. However, even the synthetic quartz glass obtained by this process undergoes rarefaction and its change in the refractive index is great in some cases depending upon the conditions of irradiation with ultraviolet lights, and the change in the refractive index of the synthetic quartz glass upon the ultraviolet light irradiation is not necessarily at a satisfactory level.